One known method for forming a copper film for ULSI micro-interconnects (damascene copper interconnects) is a process wherein a seed layer is provided by electroless copper plating, then a film of copper is deposited by copper electroplating.
However, when electroless copper plating is carried out on a mirror surface such as a semiconductor wafer, it has been difficult to obtain a deposited plating film with a sufficient adherence. Moreover, the plating reactivity is low, which has made it difficult to carry out uniform plating over the entire substrate. Up until now, when a copper seed layer has been formed by electroless plating on a barrier metal layer such as tantalum nitride, it has been difficult to uniformly form the plating, as a result of which the adhesion has been insufficient.
The inventors earlier discovered that, by adding, as an additive to an electroless copper plating solution, a water-soluble nitrogen-containing polymer having a small weight-average molecular weight (Mw) and also by either depositing catalyst metal onto a substrate to be plated or initially forming a layer of catalyst metal onto the outermost surface of the substrate prior to immersion in the plating solution and then immersing the substrate in plating solution so as to induce the polymer to be adsorbed onto the catalyst metal through nitrogen atoms, the plating deposition rate is suppressed and the crystals become very small, making it possible to form a uniform thin-film having a film thickness of 15 nm or less on a mirror surface such as a wafer (Patent Document 1). In the examples for the foregoing invention, the inventors have also shown that by initially forming a catalyst metal film onto the outermost surface of the substrate, then immersing the substrate in a plating solution so as to induce the polymer to be adsorbed onto the catalyst metal through nitrogen atoms, the plating deposition rate is suppressed and the crystals become very small, making it possible to form a uniform thin-film having a film thickness of 6 nm or less on a mirror surface such as a wafer.
In such a method, i.e., damascene copper interconnect formation, when a copper seed layer is provided by electroless plating following the formation of a catalyst metal layer, it is essential to form beforehand, and in a separate process from the catalyst metal layer, a barrier layer for preventing copper diffusion. Because this amounts to forming two layers—both a barrier layer and a catalyst metal layer—prior to forming the copper seed layer, such an approach is difficult to apply to an actual production step for ultrafine interconnects in which the film thickness cannot be made large.
Although it is desirable in electronic members for the impurities concentration within films to be made low, the level of organic impurities in films formed by CVD (chemical vapor deposition), particularly ALD (atomic layer deposition), and by electroless plating is high, with the carbon concentration exceeding 100 wt ppm. Such impurities increase resistance, hindering the uniform flow of electricity during copper electroplating and thus giving rise to irregularities and other undesirable effects.    Patent Document 1: Japanese Patent Application Laid-open No. 2008-223100